Question

Describe the start of transcription in bacteria and the function of the sigma factor. Give detailed answer.

Answer 1

Bacterial transcription is the process in which messanger RNA transcripts of genetic material in bacteria are produced, to be translated for the production ofproteins. In bacteria genetic material is not enclosed in a membrane-enclosed nucleus and has access to ribosomes in the cytoplasm.

Transcription is carried out by RNA polymerase but its specificity is controlled by sequence-specific DNA binding proteins called transcription factors.

Three main steps : Initiation, Elongation and Termination.

1. Initiation. The DNA molecule unwinds and separates to form a small open complex. RNA polymerase binds to the promoter of the template strand. In prokaryotes RNA polymerase is a holoenzyme consisting of a number of subunits, including a sigma factor (transcription factor) that recognises the promoter.
2. Elongation. RNA polymerase moves along the template strand, synthesising an mRNA molecule.
3. Termination. In prokaryotes there are two ways in which transcription is terminated. In Rho-dependent termination, a protein factor called "Rho" is responsible for disrupting the complex involving the template strand, RNA polymerase and RNA molecule. In Rho-independent termination, a loop forms at the end of the RNA molecule, causing it to detach itself.

Sigma factor :

A sigma factor (? factor) is a protein required only for initiation of transcription. It is a bacterial trancription initiation factor that enables specific binding of RNA polymerase to gene promoters.

Every molecule of RNA polymerase holoenzyme contains exactly one sigma factor subunit The number of sigma factors varies between bacterial species.

RNA polymerase holoenzyme complex consisting of core RNA polymerase and a sigma factor executes transcription of a DNA template strand. Once initiation of RNA transcription is complete, the sigma factor can leave the complex.